1) Field of the Invention
This invention relates to protective-colloid-stabilized polymer compositions in the form of their aqueous polymer dispersions or water-redispersible polymer powders comprising a film-forming base polymer and a protective colloid and also to the preparation and use thereof.
2) Background Art
Protective-colloid-stabilized polymers are used especially in the form of their aqueous dispersions or water-redispersible polymer powders in a wide variety of applications for example, as coating agents or adhesives for a very wide variety of substrates. The protective colloids usually used are polyvinyl alcohols (PVA1). To be effective, these protective colloids must have a certain molecular weight. Use is usually made of polyvinyl alcohols with a Hoppler viscosity of at least 4 mPas (4% strength aqueous solution). Polyvinyl alcohols of lower viscosity and, respectively, molecular weight do not give free-flowing powders which have no lumps and are easy to redisperse.
A disadvantage of these protective colloids is that they give relatively high dispersion viscosity and when the powders are prepared, the solids contents of the spraying mixture has to be kept low and this considerably impairs the cost-effectiveness of the process.
For preparing PVA1-containing powders, the solids contents achievable for spraying are generally up to 45% by weight, or in exceptional cases, up to 48% by weight. Solids contents above 50% for spraying cannot be processed (sprayed), due to the viscosity which rises with solids content. The solids content of the dispersion to be sprayed is generally selected so as to give a viscosity of &lt;500 mPas (Brookfield viscosity at 20 revolutions and 23.degree. C.), preferably, &lt;250 mpas. In addition, the high viscosity of the redispersion obtainable with PVA1 -containing powders is disadvantageous in many applications.
Although stabilization exclusively with low-molecular-weight compounds (e.g. emulsifiers, sugar derivatives, low-molecular-weight dextrins), in many instances gives low-viscosity dispersions and redispersions, the stabilization is generally insufficient to give powders whose redispersibility and blocking resistance compares favorably with those of protective-colloid-stabilized powders. In addition, the use of emulsifier-stabilized powders in applications with hydraulic binders gives rise to difficulties since cement stability is frequently inadequate.
In particular, when polymers are used in the form of their redispersion powders to improve the properties of mortars, a principal application sector for redispersion powders, the formulations have to remain stable for a certain time and must not change their working consistency significantly (viscosity stability or cement stability), since the user cannot be expected to prepare a new mix when a short period of time has passed.
In the concrete and mortar industry, an important role is also played by mechanical properties, such as compressive strength, porosity and therefore, air-pore content. If too many air pores are present, the compressive strength is severely reduced, and if too few or no air pores are present in the mortar or concrete, the construction material has insufficient freeze-thaw resistance. In addition, the adhesion of the hydraulically setting systems improved by adding the dispersion powder would be still better than that of systems not so improved.
EP-A 723975 (CA-A 2168157) has disclosed water-redispersible dispersion powders based on vinyl acetate-ethylene copolymers stabilized by polyvinyl alcohol. These dispersions have to be diluted to a solid content of 40% by weight before spray drying.
EP-A 770640 (AU-A 9670406) has disclosed water-redispersible dispersion powders with any desired base resin. The protective colloids used as polyelectrolytes have a charge opposite to that of the resin. The dispersion is diluted to 35% by weight solids content prior to spraying.
EP-A 725092 (CA-A 2168826) has disclosed water-redispersible dispersion powders based on styrene-acrylate copolymers. Stabilization is only by emulsifiers and sulfonic-acid-containing comonomers, and no additional protective colloid is used. The dispersion is diluted to 30% by weight solids content prior to spray drying.
EP-A 723975 (CA-A 2168157) has disclosed water-redispersible, crosslinkable dispersion powders based on styrene-acrylate copolymers. Stabilization is by polyvinyl alcohol. The dispersions have high viscosities of up to 8000 mPas at a solids content of 55% by weight and have to be diluted to a solids content of less than 50% by weight prior to spray drying.
EP-A 629650 (U.S. Pat. No. 5,462,978) describes the use of copolymers of ethylenically unsaturated monomers and of sulfonate-functional monomers, in particular acrylamido-methylpropanesulfonic acid, as a spraying aid in the spray-drying of aqueous polymer dispersions.
The object on which the invention is based is therefore to provide redispersible powders, and also dispersions for preparing the powders which avoid the above-mentioned disadvantages of the prior art. This implies a relatively low-viscosity dispersion, if desired also with a solids content of &gt;50% by weight, the powders obtainable from which give a low-viscosity redispersion and are at the same time easily redispersible, free-flowing and blocking-resistant. In particular, for application with hydraulic binders, the products should have fully satisfactory viscosity/cement stability and should not hinder the setting of cement.